COOPERATIVE AND COMPETITIVE INTERACTIONS IN THE RECRUITMENT OF MARSH ELDERS

While most studies of plant recruitment focus on competitive interacti ons, recruitment can be influenced by both positive and negative inter actions. We studied recruitment in the marsh elder, Iva frutescens, to examine the relationship between positive and negative forces in recr uitment. Marsh elder seedling establishment was competitively preclude d in undisturbed habitats by the dense perennial turfs that dominate m arsh habitats. Adult marsh elders, however, passively trapped tidally transported plant debris that kills underlying vegetation. As a result , adult Iva created bare space and positively influenced seedling recr uitment by facilitating the formation of seedling safe sites. Within b are patches, Iva seedling survivorship was high under adult conspecifi cs and at high seedling densities. In contrast, solitary Iva seedlings without adult neighbors suffered extremely high mortality. These posi tive associations among neighbors were experimentally shown to result from neighbor buffering of hypersaline soil conditions. Salinities in unshaded bare patches were elevated due to increased exposure to radia tion and surface evaporation. Both adult nurse plants and high seedlin g densities shaded the soil, ameliorated high soil salinities, and thu s had positive effects on seedling performance. Under adult canopies o r when soil salinities were experimentally reduced by shading or water ing, positive associations did not occur and all interactions among se edlings or between seedlings and adults were competitive. Our results indicate that marsh plant recruitment is dictated by a balance between positive and negative forces. Moreover, since positive interactions w ere only seen once densities and physical forces had been experimental ly manipulated, our results warn that positive interactions may be imp ortant forces in assemblages even where they are not conspicuous. Thes e findings contribute to growing evidence that positive interactions, particularly those mediated by neighbor amelioration of harsh physical conditions, are important community level processes in a wide range o f plant and animal assemblages.